Tubular prostheses typically fall into two general categories of construction. The first category of prosthesis is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter, which expands the compressed prosthesis to a larger diameter to be left in place within a vessel, e.g., an artery, at the target site. The second category of prosthesis is a self-expanding prosthesis formed from, for example, shape memory metals or super-elastic Nickel-Titanium (NiTi) alloys, that will automatically expand from a compressed state when the prosthesis is advanced out of the distal end of the delivery catheter into the blood vessel.
Some known prosthesis delivery systems for implanting self-expanding stents include an inner lumen upon which the compressed or collapsed prosthesis is mounted and an outer restraining sheath that is initially placed over the compressed prosthesis prior to deployment. When the prosthesis is to be deployed in the body vessel, the outer sheath is moved in relation to the inner lumen to “uncover” the compressed prosthesis, allowing the prosthesis to move to its expanded condition. Some delivery systems utilize a “push-pull” design and technique in which the outer sheath is retracted while the inner lumen is pushed forward. Still other systems use an actuating wire that is attached to the outer sheath. When the actuating wire is pulled to retract the outer sheath and deploy the prosthesis, the inner lumen must remain stationary, to prevent the prosthesis from moving axially within the body vessel.
There have been, however, problems associated with these delivery systems. Systems that use the “push-pull” design can experience movement of the collapsed prosthesis within the body vessel when the inner lumen is pushed forward. This movement can lead to inaccurate positioning and, in some instances, possible perforation of the vessel wall by a protruding end of the prosthesis. Further, systems that utilize the actuating wire design will tend to move to follow the radius of curvature when placed in curved anatomy of the patient. As the wire is actuated, tension in the delivery system can cause the system to straighten. As the system straightens, the position of the prosthesis changes because the length of the catheter no longer conforms to the curvature of the anatomy. This change of the geometry of the system within the anatomy also leads to inaccurate prosthesis positioning.
Systems are known for delivering or implanting a self-expanding device in a vessel by operation of a balloon to rupture a sheath that holds the self-expanding device in a compressed state. When the device is located at the desired position in the vessel, the balloon is inflated, rupturing the sheath, thereby allowing the device to expand into position. Examples of these systems include U.S. Pat. Nos. 6,656,213 to Solem and 5,549,635 to Solar.
While Solem '213 and Solar '635 describe systems for delivering a self-expanding stent by operation of a balloon to rupture a sheath, experimental implementations of systems of these types of systems have shown results that fall short of expectations. In experiments on porcine coronary arteries, withdrawal of the catheter delivery system after sheath rupturing resulted in migration of the stent from the target implant position. The amount of stent migration was unpredictable and was in the range of 3-10 mm, which is suboptimal for the treatment of coronary lesions.
There are two primary structural factors that lead to stent migration for these systems. First the stent may remain circumferentially enclosed by the sheath at points along its length even after sheath rupture, i.e., the stent may not fully exit the sheath. Secondly, the friction between the sheath and the stent during catheter removal may drag the stent proximally. The inability of these systems to offer accurate placement of a stent at a target site causes this approach to be not optimum for treatment of coronary lesions and similar stenotic disease states.
There is thus a widely recognized need for, and it would be highly advantageous to have, a delivery system that is devoid of the above limitations.